The present invention relates to a driver circuit and a data communication device used for a serial data transmission system.
As a standard for high-speed serial data transmission, IEEE P1394b (P1394b draft standard for a high performance serial bus) is known, which supports both an electric cable (metal cable) and an optical cable as a transmission route. According to this standard, once a speed negotiation cycle is terminated, a normal mode cycle for exchange of normal data is started.
In the case of communication via an electric cable, the following three states are used: the state where the potential of one signal line is high (hereinafter, referred to as “1”) and the potential of the other signal line of a differential pair is low (hereinafter, referred to as “0”); the state where the potentials of the two signal lines are the opposite of those in the above state; and the state where the inter-line voltage of the differential pair is less than a predetermined value (that is, the state where the output of a driver circuit for driving the differential pair has a high impedance).
During the speed negotiation cycle, the data transmission speed is negotiated between communication devices connected with each other via the cable. During this cycle, the driver circuit for driving the cable repeats the period during which the output of the driver circuit is kept in the high impedance state and the period during which a 50 MHZ clock is transmitted by changing the potentials of the two signal lines. During the normal mode cycle, the driver circuit transmits normal 8B10B coded data by changing the potentials of the two signal lines to “1” or “0”.
In other words, during the normal mode cycle, the driver circuit merely outputs “1” or “0”, but during the speed negotiation cycle, it has periods during which the output is neither “1” nor “0” (that is, the output is put in the high impedance state).
In the case of communication via an optical cable, an optical transceiver for driving the optical cable must be provided, and the driver circuit must drive the optical transceiver. In most cases, an optical transceiver does not accept the output of the driver circuit to which the optical transceiver is connected when the output is in the high impedance state. Therefore, in the case of performing communication via an optical cable using a data transmission system conforming to a standard such as IEEE P1394b and having the possibility that the output of the driver circuit of the system may become the high impedance state, a trouble may possibly occur when the output of the driver circuit becomes the high impedance state if the system has a structure that an optical transceiver is merely connected to the conventional driver circuit. For this reason, it is conventionally impossible to share the driver circuit for both communication via an optical cable using an optical transceiver and communication via an electric cable.